The present invention relates to an apparatus for molecular beam epitaxy, and more particularly to such apparatus well suited for automatically conveying a substrate in vacuum vessels while preventing the adhesion of flakes to the surface of the substrate.
The molecular beam epitaxy has come into the limelight in recent years as a technique for growing a thin film crystal on a substrate. In performing it on an industrial scale, however, important technical problems to be solved are left in an apparatus.
FIG. 1 is a plan view showing the construction of an apparatus which is required for performing the molecular beam epitaxy on the industrial scale (refer to `Hyomen Kagaku (Surface Science)`, Volume 3, No. 3 (1982), pp. 15-16). When broadly divided, the apparatus is composed of an introduction chamber 1, a preparation and analysis chamber 2, a growth chamber 3, and an outlet chamber 4, which are vacuum chambers, and a substrate conveying path 5 which is connected with these vacuum chambers through gate valves 7. The transfer of a substrate (not shown) between the conveying path 5 and each vacuum chamber is performed by a delivering manipulator 6 which has an arm penetrating the gate valve 7. The substrate is thrown from within the atmosphere into the introduction chamber 1. After the introduction chamber 1 has been evacuated down to an air pressure of 10.sup.-6 -10.sup.-8 Torr, the gate valve 7 is opened so as to transfer the substrate into the conveying path 5 (air pressure: 10.sup.-9 -10.sup.-10 Torr). The substrate is successively transferred into the preparation and analysis chamber 2 and the growth chamber 3 (both having an air pressure of 10.sup.-10 -10.sup.-11 Torr) through the conveying path 5, and is subjected to cleaning process of its surface and to the process of crystal growth in the respective chambers. Lastly, the substrate is conveyed out from the outlet chamber 4 (air pressure: 10.sup.-6 -10.sup.-8 Torr) into the atmosphere again.
One technical problem in the apparatus as stated above lies in a substrate conveying system under high vacuum and ultrahigh vacuum for conveying the substrate thrown from within the atmosphere into the apparatus, into the ultrahigh vacuum chambers, and for conveying the substrate subjected to the processes necessary for the epitaxial growth, one of the apparatus to the atmosphere again. The substrate must be conveyed without being damaged during the conveyance, and reliably without being dropped. Further, in order to reduce the lattice defects of an epitaxial layer, the adhesion of flakes to the substrate surface in the course of the conveyance must be avoided to the utmost.
Meanwhile, in such apparatus, a high polymer such as lubricating oil cannot be used within the vacuum vessel because the ultrahigh vacuum must be attained. Moreover, the whole vacuum tank is frequently heated to a temperature of 250.degree.-300.degree. C. for the purpose of reducing gases to be emitted from the surfaces of the components within the vessel. Under such severe conditions, a precise motion mechanism which is usually employed in the atmosphere cannot be used because it incurs the problems of the seizing between the components, rapid abrasion, etc. It is also difficult to install within the vessel a driving source such as motor and sensors such as limit switches.
For these reasons, a substrate conveying system to be stated below has heretofore been adopted (refer to P. E. Luscher, `THIN SOLID FILM`, 1981, pp. 2-12). FIG. 2 shows the principal parts of the conveying system. A substrate of low mechanical strength 8, for example, one of GaAs is stuck to or mechanically held by a susceptor 9 of high rigidity (fabricated of Mo by way of example), and the susceptor 9 is conveyed as a unit. Such susceptor 9 is transferred between the conveying path and the respective vacuum chambers while keeping the state in which the substrate plane of the susceptor stands upright.
Any of substrate holders in the introduction chamber, the preparation and analysis chamber, the growth chamber and the outlet chamber and a substrate holder to travel in the conveying path has a substrate holding ring 13 as shown in FIG. 2. The susceptor has pins 10 erected on its side surface, and is held by the substrate holding ring 13 by means of L-shaped grooves 14 in which the pins fit. The attachment and detachment of the susceptor to and from the substrate holder in each vacuum chamber, and those to and from the substrate holder in the conveying path are both performed by the substrate delivering manipulator 6. This manipulator is a mechanism by which a rod 31 located in the vacuum is rectilinearly moved and rotated from the atmosphere side. A grip portion 11 disposed on the front end of the rod has grooves 12 in which the substrate holding ring 13 and also the pins 10 of the susceptor fit. Owing to the rectilinear motion and rotary motion of the manipulator, the susceptor is delivered from each vacuum chamber to the conveying path and vice versa.
In the prior-art apparatus stated above, all the operations of attaching and detaching the susceptor are performed manually while scrupulous care is taken by visual inspection through the viewports of the vacuum vessels. Nevertheless, the susceptor is apt to fall down because of the upright attitude, and the probability of dropping the susceptor is high. In recent years, as the construction of the apparatus has progressed to have a large number of vacuum vessels, an increased number of places for transfer are included, and the probability of dropping the susceptor increases more. The dropped susceptor cannot be picked up unless the vacuum chamber is exposed to the atmospheric air, so that the substrate conveying system in the prior art has been greatly disadvantageous. Another disadvantage is that, even when the substrate surface stands upright, flakes adherent on the inner wall of the vacuum vessel cannot be perfectly prevented from dropping to adhere on the substrate surface.